Various articles, including disposable diapers, training pants, adult incontinence articles, feminine hygiene products, and the like utilize elastic or elastic-like materials to improve the conformability and/or fit of the article. However, the relatively high costs typically associated with some elastic materials may make their use in such articles undesirable. In addition, certain elastic materials may not provide suitable softness, smoothness, strength, etc. when incorporated into a wearable article. As a result, nonwoven fabrics, especially inexpensive polymer-based nonwoven fabrics, have found increasing popularity for use in disposable absorbent articles. Nonwoven fabrics are typically formed as webs, batts, mats, or sheets of fiber networks, and are sometimes referred to generally as “nonwoven webs.” An obstacle to the use of nonwoven webs in elastic textile articles has been the relatively inelastic nature of nonwoven webs formed from extensible but inelastic fibers. Therefore, laminates formed by joining a low-force stretch elastic material to one or more layers of extensible nonwoven material are typically used to reduce or even eliminate at least some of these undesirable features. In these constructions, the elastic layer may provide the elastic stretch needed for fit and comfort, while the outer nonwoven layers provide the aesthetics necessary to make the laminate feel cloth-like. However, since the nonwoven is typically not elastic, it may still offer undesirable resistance to the extensibility of the low-force stretch elastic material (i.e., “lock up” the stretch properties of the elastic material).
One way to limit the amount of lock up caused by an inelastic material such as an extensible nonwoven is to activate it. Activation, or incremental stretching as it is sometimes referred to, involves permanently stretching or elongating a web or portions of a web in one or more directions. As the web is stretched, some of the fibers, inter-fiber bonds, and/or intra-fiber bonds are believed to be broken. The breaking of the fibers and/or bonds of the nonwoven web may result in increased elasticity and/or softness in the web, at least to the degree of elongation. Known processes for activating an extensible material typically involve passing the material through one or more pairs of activation rolls. The activation rolls generally have three-dimensional surface features (e.g., teeth and grooves, peaks and channels, or corrugations), which are configured to operatively engage one another. The three-dimensional surface features on the rolls are typically complementary (i.e., fit together in an intermeshing fashion) such that the rolls are sometimes referred to as being a “matched” or “mated” pair. As the web passes through the matched pair of activation rolls, it is subjected to relatively high localized mechanical stress from the intermeshing three-dimensional surface features. Most, if not all, of the fiber/bond breaking takes place in these areas of high localized mechanical stress. Upon successful completion of the activation process, the activated web may exhibit an increase in length in one or more dimensions depending on the direction of activation.
In one known process for activating a material, a web is first fed through a pair of matched activation rolls that have raised portions (e.g., ribs or teeth) running the length of the rolls (i.e., extending in a direction parallel to the axis of rotation of the rolls or “axial direction”) to activate the material in a first direction. The axially extending raised portions of the rolls intermesh in a manner similar to the way the teeth of two gears typically intermesh. The rolls may be positioned such that the intermeshing teeth do not substantially contact one another in order to avoid damaging the teeth and/or roll. As the web passes through the pair of rolls, it is activated in the direction of travel of the material, sometimes referred to as the machine-direction (“MD”). In some instances, a matched pair of rolls may include surface features that resemble a line of alternating discs of larger and smaller diameters, sometimes referred to as a ring-rolling configuration. Ring-rolling is typically used to activate a web in the direction orthogonal to the machine direction, also referred to as the cross-direction (“CD”). In some instances, the nonactivated web may be bonded to an unstrained elastic material to form a laminate material. The laminate may subsequently be subjected to an activation process to form a “zero-strain” stretch laminate. Examples of methods of activating webs and/or methods of making elastic laminates that include activated materials may be found in U.S. Pat. No. 4,200,963 to Kamfe, et al.; U.S. Pat. No. 4,209,563 to Sisson; U.S. Pat. No. 4,525,407 to Ness; U.S. Pat. No. 4,834,741 to Sabee; U.S. Pat. No. 5,143,679 to Buell, et al.; U.S. Pat. No. 5,650,214 to Anderson et al., U.S. Pat. No. 5,156,793 to Buell, et al.; U.S. Pat. No. 5,330,458 to Buell, et al.; U.S. Pat. No. 6,476,289 to Buell, et al.; U.S. Pat. No. 6,521,555 to Bodaghi, et al.; PCT Publication Nos. WO00/029199 to Jameson, et al.; WO03/072338 to McAmish, et al.; WO05/110748 to McCormack et al.; and WO08/067463 to Middlesworth, et al.; and U.S. Publication No. 20080224351 to Curro et al. While known processes may be suitable for activating a material in one direction, it may be desirable in certain applications to activate a material in more than one direction.
Another method for providing some elasticity to a substantially inelastic, extensible material is sometimes referred to as “SELFing,” which means Structural Elastic-Like Film SELFing is similar to activation in that the material is stretched between intermeshing teeth. However, in SELFing, the teeth do not run continuously along the length of the activation area and instead are discrete along the length, leaving narrow bands of nonactivated material. In the activated areas, the material deforms permanently, while in the unactivated areas, there is substantially no permanent deformation. When a SELFed material is stretched, the material typically exhibits elastic properties.
One known method for activating a web in two directions is to pass the web through a first pair of rolls that activate the web in a first direction, and then subsequently pass the web through a second (or more) pairs of rolls to activate the web in a second (or more) direction. A problem with adding additional pairs of rolls is that it may increase the cost and complexity of a manufacturing operation. In addition, adding more pairs of rolls generally means more space is required on the manufacturing line to accommodate the additional rolls. Additionally, if the different areas of the web that are to have different directions of activation are relatively close to one another (e.g., less than 20 mm apart) it is possible that due to process tracking variability, a small portion of the web may get activated twice, once in each of two directions. This double activation may put excessive strain on the web locally, resulting in pin holes, or in extreme cases, even tears in the laminate. Thus, in at least some manufacturing processes, the additional cost, complexity and/or space requirements may make the use of additional pairs of activation rolls undesirable.
Another known method for activating a web in two directions is to pass the web through a stamping operation that uses a pair of matched plates to activate portions of a web placed between the plates. The plates may include three-dimensional surface features and are typically arranged as a complementary pair of plates with an upper plate and a lower plate. One of the plates, typically the upper plate, is moved by a piston or other means toward the other plate until the three-dimensional surface features of the plates intermesh to provide the desired depth of engagement and corresponding level of activation. The three-dimensional surface features of the plates may be configured to provide activation in multiple directions and/or to different portions of a web. However, stamping operations tend to be slower than operations that employ a roll due to the reciprocating nature of a press. In addition, the precise control required for a proper depth of engagement and strain rate may slow the process even further and/or require expensive equipment and/or complex processes. In at least some commercial industries, especially those industries where a high output of products may be required for the industry to be commercially successful (e.g., the disposable diaper industry), a stamping operation may not be a commercially desirable or even commercially viable option for activating a web at high speeds.
Accordingly, it would be desirable to provide an underwear-like stretchable outer cover for a disposable absorbent article that has different regions of activation in different directions and which has not been undesirably damaged during activation and does not have undesirable stretch lock-up.